I upgraded to the most current build 1.50 beta 3, included the source in my project, and ran it in Debug mode, where execution halted in the file PDFsharp\src\PdfSharp\Pdf.Content\CParser.cs line 163 failing an assertion:

#if DEBUG
default:
Debug.Assert(false);
break;
#endif

Without digging too deep into the analysis of PDF files, it was clear that the PDF contained a CSymbol that is not being handled by the library, and thus (most likely) ended up in an infinite loop inside CParser.ParseObject().